Fix Logger.WithField doscription
Fix Logger.WithField doscription

I was puzzled by function documentation not mentioning it works with Error level, so I had to check it out by creating example before I add logrus as a dependency on the company project. Example confirmed what logic was telling me that Logger.WithFields works with Error level of logs.
This is is a fix of this small documentation oversight.
yxd-hde/logrus,sirupsen/logrus,Sirupsen/logrus,penhauer-xiao/logrus,sirupsen/logrus,penhauer-xiao/logrus,freeformz/logrus